Re: [PATCH] Allow threads to rename siblings via/proc/pid/tasks/tid/comm

[john stultz]

On Sat, 2009-11-07 at 23:52 +0100, Andi Kleen wrote:
> john stultz <johnstul@xxxxxxxxxx> writes:
> > -	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> > +
> > +	/*
> > +	 * Threads may access current->comm without holding
> > +	 * the task lock, so write the string carefully
> > +	 * to avoid non-terminating reads. Readers without a lock
> > +	 * will get the oldname, the newname or an empty string.
> > +	 */
> > +	tsk->comm[0] = 0;
> > +	wmb();
> > +	strlcpy(tsk->comm+1, buf+1, sizeof(tsk->comm)-1);
> > +	wmb();
> > +	tsk->comm[0] = buf[0];
> 
> Is this really safe? 
> 
> reader                    writer
> 
> 
> read comm[0]
>                          set comm[0] to 0
>                          overwrites comm[1]
> read comm[1]
> read comm[2]
>                          writes comm[2] to 0 
> read comm[3]
> 
> ...
> goes beyond the end

Ah. Thanks for catching that.

Here's a reworked patch using Arjan's suggestion of memsetting the whole
string. Does this look ok to you?

thanks
-john


Setting a thread's comm to be something unique is a very useful ability
and is helpful for debugging complicated threaded applications. However
currently the only way to set a thread name is for the thread to name
itself via the PR_SET_NAME prctl.

However, there may be situations where it would be advantageous for a
thread dispatcher to be naming the threads its managing, rather then
having the threads self-describe themselves. This sort of behavior is
available on other systems via the pthread_setname_np() interface.

This patch exports a task's comm via proc/pid/comm and
proc/pid/task/tid/comm interfaces, and allows thread siblings to write
to these values.

Signed-off-by: John Stultz <johnstul@xxxxxxxxxx>

diff --git a/fs/exec.c b/fs/exec.c
index d49be6b..90003f8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -926,6 +926,15 @@ char *get_task_comm(char *buf, struct task_struct *tsk)
 void set_task_comm(struct task_struct *tsk, char *buf)
 {
 	task_lock(tsk);
+
+	/*
+	 * Threads may access current->comm without holding
+	 * the task lock, so write the string carefully.
+	 * Readers without a lock may see incomplete new
+	 * names but are safe from non-terminating string reads.
+	 */
+	memset(tsk->comm, 0, TASK_COMM_LEN);
+	wmb();
 	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
 	task_unlock(tsk);
 	perf_event_comm(tsk);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 837469a..7f59af1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1265,6 +1265,78 @@ static const struct file_operations proc_pid_sched_operations = {
 
 #endif
 
+
+
+static ssize_t
+comm_write(struct file *file, const char __user *buf,
+	    size_t count, loff_t *offset)
+{
+	struct inode *inode = file->f_path.dentry->d_inode;
+	struct task_struct *p;
+	char buffer[TASK_COMM_LEN];
+
+	memset(buffer, 0, sizeof(buffer));
+	if (count > sizeof(buffer) - 1)
+		count = sizeof(buffer) - 1;
+	if (copy_from_user(buffer, buf, count))
+		return -EFAULT;
+
+	p = get_proc_task(inode);
+	if (!p)
+		return -ESRCH;
+
+	if (same_thread_group(current, p))
+		set_task_comm(p, buffer);
+	else
+		count = -EINVAL;
+
+	put_task_struct(p);
+
+	return count;
+}
+
+
+static int comm_show(struct seq_file *m, void *v)
+{
+	struct inode *inode = m->private;
+	struct task_struct *p;
+
+	p = get_proc_task(inode);
+	if (!p)
+		return -ESRCH;
+
+	task_lock(p);
+	seq_printf(m, "%s\n", p->comm);
+	task_unlock(p);
+
+	put_task_struct(p);
+
+	return 0;
+}
+
+static int comm_open(struct inode *inode, struct file *filp)
+{
+	int ret;
+
+	ret = single_open(filp, comm_show, NULL);
+	if (!ret) {
+		struct seq_file *m = filp->private_data;
+
+		m->private = inode;
+	}
+	return ret;
+}
+
+
+static const struct file_operations proc_pid_set_comm_operations = {
+	.open		= comm_open,
+	.read		= seq_read,
+	.write		= comm_write,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+
 /*
  * We added or removed a vma mapping the executable. The vmas are only mapped
  * during exec and are not mapped with the mmap system call.
@@ -2504,6 +2576,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 #ifdef CONFIG_SCHED_DEBUG
 	REG("sched",      S_IRUGO|S_IWUSR, proc_pid_sched_operations),
 #endif
+	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
 	INF("syscall",    S_IRUSR, proc_pid_syscall),
 #endif
@@ -2839,6 +2912,7 @@ static const struct pid_entry tid_base_stuff[] = {
 #ifdef CONFIG_SCHED_DEBUG
 	REG("sched",     S_IRUGO|S_IWUSR, proc_pid_sched_operations),
 #endif
+	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
 	INF("syscall",   S_IRUSR, proc_pid_syscall),
 #endif


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